Completing wells



Oct 11, 1966 D. H. FLICKINGER 3,277,963 I COMPLETING WELLS Filed May 27,1964 DON H. FLICKINGER INVENTOR.

ATTORNEY.

United States Patent 3,277,963 COMPLETING WELLS Don H. Flickinger,Tulsa, Okla, assiguor to Pan American Petroleum Corporation, Tulsa,Okla., a corporation of Delaware Filed May 27, 1964, Ser. No. 370,735 13Claims. (Cl. 16621) This invention relates to completing wells. More particularly it relates to completing oil wells in which a metal casing iscemented through the producing zones and this casing is then perforated.

Two problems have persisted in this type of completion. First, a poorseal is sometimes formed between the casing and cement. This results ina flow passage through which fluids above or below the producingformation can flow down or up the well outside the casing to theperforation through which the desired fluid enters the casing. Second,the casing perforations may be so plugged with debris that fluids canflow through the perforations only with difficulty, if at all. Thesecond problem is particularly important in locations where theformation is not fractured after the perforations are formed.

An object of my invention is to provide a method for forming a betterbond between the casing and the cement outside the casing. Anotherobject is to provide a method for forming casing perforations withhigher flow capacities. Still other objects will be apparent to thoseskilled in the art from the following description and claims.

In general I accomplish the objects of my invention by running emptytubing into the well, the tubing having a means for closing the bottomof the tubing to prevent entry of liquids. A packer is attached to thelower part of the tubing string. The tubing is run into the well untilthe packer is a short distance above the top producing zone in the well.The packer is then set and the closing means at the bottom of the tubingis opened to release the pressure in the bottom portion of the casing.

In a cementing operation these steps are taken immediately after cementslurry has been placed behind the casing. Release of pressure inside thecasing in this case allows the pressure outside the casing to compressthe casing and reduce its circumference. The cement then sets around thecasing in this condition. Thus, when the mud or other liquid is removedfrom the well, the casing does not shrink away from the cement due tothe decrease of pressure inside the casing to provide a space betweenthe casing and the cement outside the casing.

If the technique is used in perforating, the decrease of pressure hasbeen shown to decrease the amount of debris in the perforations. Theexact reason is not certain. It is probably due to the lower peakpressure caused by the explosive used in the perforator. Any decrease inthis peak pressure should also decrease the rapid flow of liquids anddebris into the perforations. It is also possible that the rapid backflow of fluids through the perforations from the high pressure formationto the low pressure well may simply remove much of the debris leaving aclean perforation.

In the drawing, FIGURE 1 is a view in cross section of the apparatusused in the preferred embodiment of my in vention. FIGURE 2 is anothercross-sectional view of an alternate sealing mechanism for ports in thetubing.

Considering the drawing in more detail: in FIGURE 1 well casing extendsinto a well penetrating oil producing formation 11 and nonprodueingformations 12 and 13. On the bottom of casing 10 is casing shoe 14- withcheck valve ball 15. The casing has been cemented by displacing a cementslurry 16, shown surrounding the casing, down the casing by means ofdrilling fluid, water or the like. The cement slurry is separate fromthe following liquid by cementing plug 17.

Tubing 18 extends into the casing to a level near the top of the cementcolumn behind the casing. The bottom of the tubing is closed byfrangible disc 28 {which is held in place by cap 19. A packer 20 sealsthe space between the tubing and casing.

In the method of my invention the casing is run into the well as usualand the cement slurry is displaced down the inside of the casing and uparound the outside. Preferably, solids-free water is used for thisoperation if the casing is to be perforated before removal of theliquid. Tubing 18 with packer 20 is then run into the well as quickly aspossible. The tubing is empty. Packer 20 is set to form a seal betweenthe tubing and casing at or near the elevation of the cement outside thecasing or at least above the level of the top producing zone in theformations penetrated by the well. A go-devil (not shown) is thendropped down the inside of the tubing to break frangible disc 28. Whenthe disc is broken, the pressure inside the casing below the packerdrops to the level of the pressure in the tubing. This pressure issubstantially atmospheric. Therefore, the pressure within the casingbelow packer 20 alsobecomes substantially atmospheric, except for thehydraulic head of liquid below the packer. The pressure outside thecasing will be approximately the pressure of fluids in the formation.The higher pressure outside the casing will cause the casing tocontract. That is, the circumference and diameter of the casing willdiminish. If the operations leading to the reduction of pressure belowthe packer are carried out quickly enough, the cement slurry outside thecasing will not have taken a final set. Therefore, as imposition of thedifferential pressure causes the casing to shrink, cement slurry followsit and completely fills the space between the contracted casing and thewell wall. The cement is then permitted to set while maintaining thereduced pressure inside the casing.

After the cement has set, packer 20 is released and tubing 18 iswithdrawn from the well. The remainder of the completion of the well canthen be in any usual manner. When packer 20 is released, the fullhydraulic head of liquid in the casing will again be applied inside thecasing below the packer. The casing will not be able to expand, however,since it is supported on the outside by the hardened cement which is, inturn, supported by the formation. During the depletion of the formationin which the well is completed, the pressure inside the casingdecreases. It is this decrease in pressure which in the past has oftencaused the casing to contract and pull away from the set cement sheathoutside the casing. The result has been a flow path between the casingand the cement. In my method, however, the casing has been caused tocontract before rather than after the cement sets. Therefore, no spacecan be left between the casing and cement when the pressure inside thecasing is reduced during the life of the well.

In FIGURE 2 tubing 18 is the same as in FIGURE 1, In this case, however,ports 21 are provided in the wall of the tubing below packer 20. Thebottom of the tubing can be closed in this case by a permanent plug, capolr the like. The bottom of the tubing can also carry a perforator ifdesired as described later in more detail. Ports 21 are sealed bysealing elements 22 such as O-rings around an open-ended tubular body23. Preferably a small shear pin 24 is provided through. the wall of thetubing and extended into body 23. This pin holds the body with its sealsin positicin over ports 21 while the tubing is being lowered into thewell. Retrieving head 25 is provided on the top of the tubular body.

When the apparatus of FIGURE 2 is used, ports 21 are opened by loweringa wire line into! the well with a retrieving tool to latch around head25. An upward pull on the wire line then shears pin 24 and permitsremoval of body 23 from the tubing. To facilitate removal of body 23from the well, that portion of the tubing a short distance on each sideof ports 21 may be of slightly smaller internal diameter than theregular internal diameter of the tubing. Thus, the O-rings can seal inthe small-diameter portion but not contact the inside wall of theregular tubing. Opening ports 21 has the same effects as described forbreaking frangible disc 28 in connection with FIG- URE 1.

If the apparatus of FIGURE 1 is used, a perforator can be loweredthrough the tubing after the cement is set and the casing can beperforated opposite the producing formation. The pressure in the casingbelow the packer is still low and the perforation step takes advantageof this fact to produce perforations with higher flolw capacities thanthose which would be made with high pressures inside the casing.

If the apparatus of FIGURE 2 is used and a perforator is placed on thebottom of the tubing opposite a producing zone, this perforator can befired by any of the well known means after the cement has set.

It will be apparent that the technique can be applied to improvingcementing even if the techinque is not used for perforating the casing.For example, if the formation is to be subjected to hydraulicfracturing, the flow capacities of the perforations in the formation arenot particularly important. The flow capacities of the holes through thecasing wall will, of course, be important. The flow capacity of thefracture in the formation will be so much greater than the flowcapacities of any perforations in the formation that these flowcapacities become unimportant. Thus, after using my technique incementing, a horizontal casing-cutting shaped charge, as shown in US.Patent 2,939,532, for example, can be used to form a large cut throughthe casing through which hydraulic fracturing operations can be carriedout. Such an explosive charge would not, on the other hand, be used inareas where wells ordinarily are not fractured. In these cases, the flowcapacities of the perforations are very important. The casing cuttingcharge sometimes does not even penetrate the cement sheath. Thus,penetration into the formation is much too small to provide a high flowcapacity into the well. Where the perforating step is not to be followedby further formation treatment, it will be obvious that :a bullet orlinear jet perforator should be used with a low pressure inside thecasing.

The cementing technique can also be used to set casing above a producingformation before drilling into the producing formation for an open-holecompletion. In this case, no perforation is usually considerednecessary.

It will also be apparent that the perforating step can be carried out atreduced pressure by use of my technique even though the casing has beencemented by other methods. Thus, the perforating technique can beapplied to new or old wells in which standard cementing processes havebeen used. It is preferred, however, to combine the cementing andperforating operations as described above.

Many types of specific apparatus may be used as long as they include apacker which can be set in the casing to isolate a portion of the casingbelow the packer, and a means for decreasing the pressure inside thecasing below the packer. For example, apparatus used in well testing canbe used. Such apparatus frequently includes a packer which can be set incasing and a chamber which can be opened to take a sample of fluids frominside the casing below the packer. Opening of the sample chamber allowsexpansion of the liquid in the casing into the chamber thus reducing thepressure. Since the liquid has a low coefficient of expansion, a smallvolume, a gallon or two for example, will provide adequate expansionvolume to drop the pressure to a point near the original pressure in thesample chamber.

The same general type of equipment can be used if the method is toinclude a perforating step. The only difference is that a perforatormust be provided in the zone below the packer or provisions must be madeto introduce such a perforator. One class of apparatus known generallyas perforate and test equipment is particularly useful. Such apparatusis shown, for example, in US. Patent 2,535,342, Ahlgren, 2,601,122,McKinley, and 2,760,541 Johnston et al. It is only necessary to modifysuch equipment so the sample chamber is opened below the packer beforethe perforator is fired rather than after the perforator is fired, asproposed in the patents.

In all the methods described above, a packer is set in the casing andthe pressure inside the casing below the packer is decreased. No limitshave been set with regard to the amount of pressure reduction. In mostcases the pressure is decreased as much as conveniently possible. In thepreferred embodiment, for example, the tubing is run into the well emptyand the packer is set only a few feet, for example, a few hundred feet,above the top producing zone. Thus, when the inside of the tubing isconnected to the space inside the casing below the packer, the pressureat this point is reduced to substantially atmospheric pressure. If thecementing operation is to be followed by perforation of the casing atlow pressure and the formation is known to be at high pressure, however,it may be advisable to fill the tubing partly with liquid to helpcontrol the well after the perforators are fired. A higher pressure canalso be maintained below the packer by simply setting the packer aconsiderable distance, such as a thousand feet or so, up the well fromthe top producing formation,

The use of higher pressure, but still below the pressure outside thecasing, is particularly desirable in the cementing operation if it isknown that the inside of the casing is later to be subjected to highinternal pressures as in a hydraulic fracturing operation. In any casethe packer should be set a substantial distance, at least severalhundred feet, below the top of the well. The tubing should be at leastpartly empty leaving at least several hundred feet empty at the top.

In general, in the perforating operation, the greater the pressurereduction the greater the advantage. A small reduction in pressure belowthe packer will provide some benefits. The major improvement isprovided, however, by reducing the pressure inside the casing to a valueless than that outside the casing at the time of the perforation.

In the perforating operation timing is not important as long as thecement is allowed to set sufficiently before the perforating takesplace. In the cementing operation, however, it is essential that thepacker be set and the pressure be reduced before the cement slurry hastaken its final set. The hydraulic head of liquids outside the casing isordinarily great so as large force is available to cause the cementslurry to deform and fill the complete space between the casing and wellwall as the casing contracts. If a final set has not been reached whenthe pressure inside the casing is decreased, there is little questionthat sufiicient deformation of the cement slurry will take place.Preferably, the pressure is to be reduced as quickly as possible toprovide the greatest assurance of success.

The above-described methods and apparatus are given by way of exampleonly. I do not wish to be limited to the specific methods and apparatusbut only by the following claims.

I claim:

1. A method for completing a well comprising lowering a casing stringinto the well, said casing string having a check valve on the bottom toprevent backflow, introducing cement slurry down said casing and forcingsaid slurry up around the outside of said casing by introducing a liquidinto said casing after said cement slurry, lowering a packer asubstantial distance into said casing to a level above the highestproducing formation of interest penetrated by the well, setting saidpacker in said casing to isolate a zone inside the casing below saidpacker, opening a low pressure expansion chamber to the Zone inside thecasing below said packer to reduce the pressure within said casing belowsaid packer before said cement slurry takes a final set, and maintaininga reduced pressure within said casing below said packer until saidcement slurry takes a final set.

2. The method of claim 1 in which said casing is perforated with alinear perforator opposite at least one producing formation after saidcement slurry has set but while asid reduced pressure is maintained insaid casing below said packer.

3. The method of claim 2 in which said liquid introduced after saidcement slurry is solids-free water.

4. The method of completing a well in which a casing string is cementedthrough a producing formation comprising lowering a packer into saidcasing to a level substantially below the top of said well but above thetop of said producing formation, setting said packer to isolate a zoneinside the casing below said packer, opening a low pressure expansionchamber to the Zone inside the casing below said packer to reduce thepressure in the easing below said packer, and firing a linear perforatoropposite said formation at said reduced pressure, whereby high flowcapacity perforations are formed through said casing and into saidformation.

5. The method of claim 4 in which the pressure inside said casing belowsaid packer is decreased to a value below the pressure outside saidcasing before firing said perforator.

6. A method of completing a well comprising lowering a casing stringinto the well, said casing string having a check valve on the bottom toprevent backflow, introducing cement slurry down said casing and forcingsaid slurry up around the outside of said casing by introducing a liquidinto said casing after said cement slurry, lowering a packer asubstantial distance into said casing to a level above the highestproducing formation of interest penetrated by the well, said packerbeing lowered on a tubing string closed at its lower end and at leastpartly empty, setting said packer between said casing and tub ing,connecting the inside of said tubing to the inside of said casing belowsaid packer to decrease the pressure inside said casing below saidpacker before said cement slurry takes a final set, and maintaining areduced pres sure inside said casing below said packer until said cementslurry takes a final set.

7. The method of claim 6 in which said casing is perforated opposite atleast one producing formation after said cement slurry has set but whilesaid reduced pressure is maintained in said casing below said packer.

8. The method of completing a well in which a casing string is cementedthrough a producing formation comprising lowering a packer into saidcasing to a level substantially below the top of said well but above thetop of said producing formation, said packer being lowered on a tubingstring closed at its lower end and at least partly empty, setting saidpacker between said casing and tubing, connecting the inside of saidtubing to the inside of said casing below said packer to decrease thepressure inside said casing below said packer and firing a linear 6perforator opposite said formation at said reduced pressure, wherebyhigh flow capacity perforations are formed through said casing and intosaid formation.

9. A method for completing a well comprising lowering a casing stringinto the well, said casing string having a check valve on the bottom toprevent backfiow, introducing cement slurry down said casing and forcingsaid slurry up around the outside of said casing by introducing a liquidinto said casing after said cement slurry, lowering a packer asubstantial distance into said casing to a level above the highestproducing formation of interest penetrated by the well, said packerbeing lowered on a tubing string which is at least partly empty, saidtubing string having below said packer an opening closed by a removablemember, setting said packer between said casing and tubing, removingsaid removable member from said opening to decrease the pressure insidesaid casing below said packer before said cement slurry takes a finalset, and maintaining a reduced pressure inside said casing below saidpacker until said cement slurry takes a final set.

it The method of claim 9 in which said casing is preforated opposite atleast one producing formation after said cement slurry has set but whilesaid reduced pressure is maintained in said casing below said packer.

11. The method of claim 9 in which said opening is the bottom end of thetubing and said removable member is a frangible disc.

12. The method of claim 11 in which a linear perforator is loweredthrough said tubing and is fired opposite at least one producingformation after said cement slurry has set, while the pressure insidesaid casing below said packer remains at a reduced value.

13. The method of completing a well in which a casing string is cementedthrough a producing formation comprising lowering a packer into saidcasing to a level substantially below the top of said well but above thetop of said producing formation, said packer being lowered on a tubingstring which is at least partly empty, said tubing string having belowsaid packer an opening closed by a removable member, setting said packerbetween said casing and tubing, removing said removable member from saidopening to decrease the pressure inside said casing below said packer,and firing a linear perforator opposite said formation at said reducedpressure, whereby high fiow capacity perforations are formed throughsaid casing and into said formation.

References Cited by the Examiner UNITED STATES PATENTS 2,798,558 7/1957McCulloch 166115 2,805,722 9/1957 Morgan et al. 166-35 2,833,352 5/1958Lloyd 16635 X 2,895,554 7/1959 Krueger et al. 166-35 3,183,971 5/1965McEver et al. 16621 3,195,631 7/1965 Smith 166-35 JACOB L. NACKENOFF,Primary Examiner.

CHARLES E. OCONNELL, Examiner.

S. J. NOVOSAD, Assistant Examiner.

1. A METHOD FOR COMPLETING A WELL COMPRISING LOWERING A CASING STRINGINTO THE WELL, SAID CASING STRING HAVING A CHECK VALVE ON THE BOTTOM OFPREVENT BACKFLOW, INTRODUCING CEMENT SLURRY DOWN SAID CASING AND FORCINGSAID SLURRY UP AROUND THE OUTSIDE OF SAID CASING BY INTRODUCING A LIQUIDINTO SAID CASING AFTER SAID CEMENT SLURRY, LOWERING A PACKER ASUBSTANTIAL DISTANCE INTO SAID CASING TO A LEVEL ABOVE THE HIGHESTPRODUCING FORMATION OF INTEREST PENETRATED BY THE WELL, SETTING SAIDPACKER IN SAID CASING TO ISOLATE A ZONE INSIDE THE CASING BELOW SAIDPACKER, OPENING A LOWER PRESSURE EXPANSION CHAMBER TO THE ZONE INSIDETHE CASING BELOW SAID PACKER TO REDUCE THE PRESSURE WITH-